1. Field of the Invention
This invention relates to coal liquefaction and is particularly concerned with liquefaction processes wherein coal liquids produced by the extraction of coal with a hydrogen-donor solvent are subsequently hydrogenated for the production of solvent and additional liquid products.
2. Description of the Prior Art
A number of different processes have been developed for the production of liquid hydrocarbons from coal. Among the most promising of these are processes in which the feed coal is first contacted with a hydrogen-containing gas and a hydrogen-donor solvent at elevated temperature and pressure in a liquefaction reactor and a portion of the liquid product is then catalytically hydrogenated in a solvent hydrogenation reactor to generate additional liquid products and solvent for recycle to the liquefaction step. Hydrogenation of the liquid is generally carried out at a pressure substantially the same as that in the liquefaction reactor and at a somewhat lower temperature. To supply the heat required for hydrogenation of the liquid, all of the vaporous product taken overhead from the liquefaction reactor is passed directly to the solvent hydrogenation reactor without cooling. The liquid effluent from the liquefaction step is normally passed to a low pressure separator in which gases are taken off and then fractionated, the lighter constituents being employed as feed to the solvent hydrogenation reactor and the heavier material being upgraded by hydrogenation, catatytic cracking, coking or the like. Hydrogen in addition to that contained in the vapors taken overhead from the liquefaction reactor is supplied to the solvent hydrogenation reactor by separating gases from the hydrogenation reactor product, scrubbing these gases to remove ammonia and other constituents, and then recycling the remaining gas with makeup hydrogen to the hydrogenation reactor. A portion of this recycle stream is also recycled to the liquefaction reactor. Liquid products from the solvent hydrogenation reactor are fractionated following recovery of the gas, the lighter constituents being taken overhead for use as fuel or the like and the heavier constituents being recycled for use as solvent or recovered as additional product.
Although the process outlined above has numerous advantages over other liquefaction processes, studies and experimental work have shown that the catalyst employed in the solvent hydrogenation reactor tends to lose activity and must be regenerated or replaced at relatively frequent intervals. In addition, substantial quantities of makeup hydrogen are required in the process and relatively large volumes of gas must be purged from the recycle gas stream to maintain the required hydrogen partial pressure in the system. These and associated problems add significantly to the operating cost of the process and tend to restrict its application.